1. Field of the Invention
The present invention relates to a method of ultrasonic inspection of titanium alloy articles in the as-cast condition, and more specifically the ability to detect sub-surface flaws without the need of significant processing to improve ultrasonic inspectability.
2. Description of the Prior Art
The use of titanium alloys for many critical structural applications has resulted in the development and use of a variety of inspection methods. These methods can be classified into volume methods that allow interrogation of the interior (subsurface) of the material and surface methods that permit detection of surface anomalies. These methods are complementary in nature and used concurrently to achieve a high confidence level in detecting undesired conditions that could compromise properties in the material or component. Surface defects are more common, but also are easier to detect and, therefore, catastrophic failure due to surface defects is less likely. Failures from internal defects, on the other hand, are obviously of greater concern compared to surface defects. The ability to consistently find small internal defects has improved the reliability of high performance structures and has led to reductions of unexpected service failures. The structural efficiency of these components also has increased because of the ability to design to higher operating stresses without increasing the risk of unexpected failure.
Ultrasonic inspection of titanium and titanium alloys is the most common inspection method used when the material is intended for use in high performance applications such as the aerospace and energy industries. In this inspection method, ultrasonic waves are induced in the material using a piezoelectric transducer. The transducer is coupled by water or other coupling media to the piece being inspected. The detection of subsurface defects is based on the reflection of some of the incident ultrasonic waves from regions lying along their path. This reflection occurs whenever there is a region that has different acoustic impedance or resistance to the transmission of the ultrasonic waves. During operation, the transducer sends waves, stops sending and waits to detect the reflected waves. There always is a reflection from the front and rear faces of the piece being inspected, which are useful length markers to help physically locate sources of other reflections along the ultrasonic pathway.
Ultrasonic testing typically requires that items to be detected possess high acoustic reflectance behaviors from bulk material under ultrasonic inspection. This different behavior permits the ultrasonic inspection technique to confidently detect flaws and imperfections. Materials with large, elastically anisotropic grains, such as, but not limited to, cast ingots of steels, titanium alloys and nickel alloys, are often difficult to evaluate by ultrasonic testing. The difficulties arise, at least in part, because sound waves, which are used for ultrasonic inspection, can be partially reflected from grains, and represent a background “noise.” The generated background noise can mask flaws in the material, and is thus undesirable. The scattering of sound in a polycrystalline metallic material body, which is also known as attenuation of a propagating sound wave, can be described as a function of at least one of the following: grain dimensions, intrinsic material characteristics, and ultrasound frequency. Use of focused ultrasonic beams to enhance a flaw fraction within any instantaneously insonified volume of material is common. These developed ultrasonic inspection techniques can identify indications based both on maximum signal, as well as signal to noise. However, if the noise level is high, which is the case with coarse grain materials, reliable detection of internal flaws using ultrasonics is not possible.
Titanium ingots in the as-cast condition exhibit extremely coarse grains, in the range of several millimeters to centimeters. These grains follow solidification patterns and are “noisy,” which implies that frequent, low amplitude reflections are observed during ultrasonic inspection. In the extreme, this noise gives rise to false positives or insufficient inspection sensitivity necessary to meet the detectability requirements. The most effective solution to this situation is to process the ingots to refine grain structure. Several steps of hot working (repeated heating and mechanical working) to refine grain structures is the standard practice to accomplish this objective. However, this processing is significantly expensive and time consuming. Intermediate products such as billets are routinely inspected ultrasonically to assess whether its quality is suitable for the final processing and eventual service. These intermediate products are products which have already undergone the above-noted hot working before the ultrasonic inspection is performed.
There is a need for an improved approach to be able to reliably inspect titanium billets in the as-cast condition. The improved approach should permit detection of internal flaws with low interference from noise, and also be compatible with subsequent processing of the billets into articles. The present invention fulfills this need, and further provides related advantages.